People are paying more and more attention to the use of the power generation of renewable resources such as wind power generation, solar power generation, and tidal power generation. Common features of power generation systems of such new resources lie in dispersive power generation devices, small unit capacity, wide distribution area, and unstable output voltage and current. It has been currently an urgent problem to be solved in our country and worldwide on how to feed the power generated by the power generation devices of such renewable resources back to power networks efficiently, reliably at a low cost, so that the power generated by the power generation devices can be converted into the three-phase power for industrial and domestic use directly.
An AC excited wound rotor double-fed generator variable-speed constant-frequency wind power generation system is adopted in a power feedback device for wind power generation in the prior art, and a power converter at the rotor side is adopted in this system to adjust the AC excited current of the double-fed generator so that the stator winding of the generator generates power which is directly fed back to power networks. Due to the feature of double-fed generator systems, a power converter with low voltage and operating in four quadrants are generally desired, for example, an AC-DC-AC two-level converter that can operate in four quadrants. FIG. 1 illustrates the principle of an AC-DC-AC two-level frequency converter that can operate in four quadrants in the prior art. As shown in FIG. 1, the frequency converter according to this solution only processes slip power, and general rated power is one third of the capacity of the generator and also falls into a low voltage converter. Thus, the cost and volume of the converter is greatly decreased, while this solution has disadvantages that the volume and cost of the generator is increased as wound rotor is adopted in the generator and AC excitation are conducted via slip ring, and the generator has a high failure rate and maintenance fee due to the use of the slip ring.
A permanent magnetic generator variable-speed constant-frequency wind power generation system is adopted in another power feedback device for wind power generation in the prior art. In this solution, a fan impeller makes the permanent magnetic generator to rotate, and the power generated is changed into three-phase AC power matching the power networks after being variable-frequency modulated by the power converter and is then fed back to the power networks, realizing variable-speed constant-frequency power generation. FIG. 2 illustrates the principle of the variable-speed constant-frequency wind power generation system of a permanent magnetic generator in the prior art, and FIG. 3 illustrates the principle of another variable-speed constant-frequency wind power generation system of a permanent magnetic generator in the prior art. As shown in FIGS. 2 and 3, this solution solves the problem of reliability of the generator in the above solution, and the whole system runs at a lower failure rate, but the converter has a high cost and the converting device has a large volume as in this solution the converter power is the same with the generator power and many electrolytic capacitors need to be used. FIG. 4 illustrates the principle of a variable-speed constant-frequency wind power generation system in which a current type converter is adopted in the prior art. As shown in FIG. 4, a semi-controllable power semiconductor device thyristor is adopted in this system, although this solution has low cost, it has serious harmonic pollution at the network side, and the power factor is low, and harmonic treatment device need to be added additionally, which raises the overall cost.